Lithium-Ion Batteries-Energy Storage: Comparison
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Please note this is a comparison between Version 2 by Camila Xu and Version 1 by Nikita Martyushev.

Lithium-ion (Li-ion) batteries are a good energy storage solution for plug-in electric vehicles. However, the performance and health of these batteries is highly dependent on the use case, including operating temperature, power consumption profile, and control strategy (heavy forced alternating charge–discharge modes) imposed by the battery management system.

  • Lithium-ion
  • storage batteries
  • Positive Electrodes

1. Li-Ion Battery

Lithium power sources can be divided into several types according to the materials used in their production and the technologies implemented.

1.1. Positive Electrodes

Since carbon (graphite) which is most commonly used as a negative electrode doesn’t bear any Li-ions, the positive electrode must act as the source of Li. Thus, the cathodes are generally intercalation compounds from which Li+ ions can diffuse out or back in. High cathode voltage is generally desirable as energy stored in the cell is directly proportional to it. However, electrolyte stability also has to be considered. A number of candidates have been explored as suitable candidates for cathode materials of Li-ion batteries, and can be categorised based on voltage verses lithium. The 2 V cathode materials are TiS2 and MoS2 with 2-D layered structure; 3 V cathode materials are MnO2 and V2O5; 4 V cathode materials are LiCoO2, LiNiO2 with 2-D layered structure and 3-D spinel LiMn2O4 and olivine LiFePO4; 5 V cathode materials are olivine LiMnPO4, LiCoPO4, and Li2MxMn4-xO8 (M = Fe, Co) spinel 3-D structure.
In 1996, researchers at the University of Texas in Austin found that phosphate materials were well-suited to be used in Li-ion battery positive electrodes. LiFePO4 (LFP) offers a number of advantages, such as great electrochemical performance, high current rating, stability, excellent cycle life [49][1], and temperature tolerance (243 K to 333 K), which makes it less prone to be suffering from a thermal runaway. The phosphate structure provides stability to the electrode against overcharging and provides higher tolerance to heat, limiting the breakdown of the material.
However, LFP has a disadvantage of poor electronic and ionic conductivity as well as relatively low capacity [50][2]. Moisture seems to significantly limit the lifetime of the battery. In addition, it effectuates diffusion of ions through one-dimensional channels, which can get blocked very easily by defects and impurities. Nevertheless, as LFP holds decent rate capabilities, extensive research is being carried out to limit down the issues. To enhance conductivities, the two common strategies are employed, namely doping by ions and coating by carbon [51][3]. Combined doping and nanoscale size, it was possible to produce high power Li-ion batteries based on LiFePO4.
Currently, LiCoO2 is the most widely used in commercial Li-ion batteries as it is easily manufactured in large scale and is stable in air, deintercalating and intercalating Li around 4 V. They have a high energy density and high cyclability [52][4]. However, lithium cobalt batteries are highly reactive due to the occurrence of Li-plating on rapid charging, consequently suffering from poor stability and must be monitored during operation to ensure safe use. Furthermore, Co is toxic and not sufficient cobalt is not globally available to meet perceived demands, making it expensive for exploitation of rechargeable batteries in EVs.
Solid solutions of Li, NiO2 with Co, Fe, Mn, Al, Ti and Mg were developed in order to develop enhanced chemistry that providing high stability along with supporting stable structure. The lithium Nickel Manganese Cobalt Oxide (NMC) electrode was developed from such solutions. They are designed for high specific energy with high density that is achieved due to the presence of nickel, and the low stability provides by it is managed by the aspinel structure formed due to the presence of Mn. The stochiometric ratio of the metals in the compounds is a very closely guarded formula. NMC the most successful Li-ion system and is suitable for EV power trains.
Li-manganese batteries (LMO) are often blended with NMC to improve specific energy and prolong life span. The LMO-NMC has been used by multiple EV manufacturers in the past including Nissan Leaf, Chevy Volt and BMW i3. Developing composite electrodes using structurally integrated layered Li2MnO3 and spinel LiMn2O4, with a chemical formula of xLi2MnO(1-x)Li1+yMn2-yO4 is one of the main fields of research regarding cathodes of Li-ion batteries. A rechargeable capacity in excess of 250 Ahkg−1 was reported in 2005 using this material, which has nearly twice the capacity of currently commercialized rechargeable batteries of the same dimensions and are expected to play increasing roles in commercial Li-ion batteries.

1.2. Negative Electrodes

Carbon anode has been dominant in the Li-ion battery industry since its commercialization as it renders excellent cyclability by facilitating the movement of lithium ions in and out of its lattice space with minimum irreversibly [53][5]. Carbons that are capable of reversible lithium-ion storage can be classified as graphitic and non-graphitic (dis-ordered) carbon. Graphitic carbons have a layered structure. The lithium insertion into graphite follows a stepwise occupation of the graphene interlayers at low concentrations of the lithium ions, known as stage formation [54][6]. Non-graphitic carbons consist of carbon atoms that are arranged in a planar hexagonal network without an extended long-range order with crystalline graphitic flakes cross-linked by the amphorous domains. They have high specific capacity but also hold issues of capacity fading and huge irreversible capacity loss from the first cycle, which may be attributed to the formation of a passivating solid electrolyte interphase (SEI) on the carbon surface. SEI may grow with time increasing the cell resistance and subsequently decreasing the cell energy density. Ongoing studies focus on the exact mechanism by which high-specific capacity is achieved in disordered carbon. Other carbon-based materials that have been studied are the buckminsterfullerene carbon nanotubes, and graphene. Carbon nanotubes serve as great hosts for Li due to their linear dimensionality [55][7] and good conductivity. Graphene-based composites have a very high capacity [56][8].
Silicon based anode is recently receiving much attention lately due to its high theoretical capacity, about 10 times that of graphite and 4 times that of spinel oxides. However, silicon suffers two main drawbacks, namely poor conductivity and huge volume variation (400%) upon cycling, which limits the use of bulk silicon anode. The advantages of silicon can still be exploited by replacing bulk Si by Si nanostructured anode materials [57][9]. Si nanowires show efficient reversible lithium storage properties, and enhanced cyclability and rate performances. However, producing Si nanomaterials is not yet cost effective to be produced in large scale. If that issue is resolved, then Si nanowires hold the potential to replace carbon anodes. Since nanomaterials have comparatively low energy density, their application would be limited to devices with no space restriction. To produce Si/C nanocomposites with optimised ratio of Si to C to obtain a trade-off between specific capacity and energy density, further studies are necessary.

1.3. Electrolyte

The electrolyte is the solution compromising the salts and the solvents and is the third component of the battery. The choice of electrolyte is very crucial and it determines the stability of the cell. The electrolyte is supposed to be chosen such that it can withstand the redox environment at both cathode and anode sides and the voltage range involved without decomposition or degradation. A positive electrode that’s highly oxidising, for instance, will require electrolyte combinations that operate well outside their window of thermodynamic stability. Liquid electrolyte in commercialised Li-ion batteries are typically a solution of Li salts in organic solvents. The existing liquid solvent, however, has issues and needs enhancement. Ideally, the electrolyte should be environmentally benign and cost effective. Currently, polar aprotic organic solvents, such as carbonate solvents with high dialectic constant, are selected to solvate lithium salts at a high concentration (1 M typically).
Other type of electrolytes used are the polymer electrolytes, gel electrolytes and ceramic electrolytes. Polymer electrolyte contain lithium salts dissolved in polymers with typically high molecular weight [58][10]. They offer numerous advantages over liquid electrolytes such as improved safety properties due to low volatility, design flexibility, cost effectiveness, potential to eliminate additional separators, ease of processing [59][11]. One of the widely studied polymer is poly(ethylene oxide), which has been coupled with various lithium salts, such as LiCF3SO3 and LiClO4 and operated in amorphous phases and has good mechanical and electrochemical stability. However, the ionic conductivity of the polymer electrolyte is significantly lower than that of the liquid electrolyte. Another example is LiTFSI58, which is stable, non-toxic and has higher ionic conductivity.
Ceramic electrolytes are attracting much research focus lately. It contains no flammable organic solvents and is thus safer than the other categories of electrolytes in high temperature medium. Another interesting property of these electrolytes is that their ionic conductivity increases with increasing temperature due to the formation and flow of ionic point defects [60][12]. Research on polymer electrolyte is aimed at achieving high conductivity at room temperature. With that, polymer electrolytes could find implementation in the next generation Li-ion batteries used in electric vehicles due to its excellent abuse tolerance.

1.4. Separator

Optimising the chemical stability of any electrode-electrolyte requires a control over the electrode-electrolyte interface through surface chemistry. Thus, separators are essential components of Li-ion batteries. The separator in a Li-ion battery plays the critical roles to avoid direct physical contact between the cathode and anode, and prevents short circuit to occur. At the same time, the separator allows lithium ions in the electrolyte to pass through it. The separators must be chemically stable and inert in contact with both electrolyte and electrodes.
The separator has to be chemical stable and inert in the given electrolyte and electrode system. However, it must be mechanically robust to withstand the tension and puncture by electrode materials.
Although various separators, including microporous polymer membranes, nonwoven fabric mats and inorganic membranes have been explored, the microporous polyolefin materials-based polymer membranes are dominantly used in commercial Li- ion batteries with liquid electrolyte. They can be constructed to be extremely thin and highly porous such that they have higher conductivity, at the same time sustaining its mechanical robustness. Microporous membranes also provide precaution against thermal runaway or short-circuiting of the battery through properly designed multilayer composites, which works such that one layers melts to close the pores and another part provides mechanical strength and electrical insulation. One example of a separator made with this technology is the microporous separator made of both polyethylene (PE) and polypropylene (PP), in the form of tri-layer of PP- PE- PP. The melting points of PE and PP are 408 K and 438 K, respectively. In the case of temperature rising due to overheating, the porosity of the membrane could be closed by PE, preventing further reactions, while the PP later stays intact to retain support and insulation. Since the separator cost constitutes a significant fraction of the total expenses of the battery, efforts are directed on trying to produce efficient separators at lower cost [61][13].

2. Comparison of Different Types of Batteries

An important feature of LIB is its short charging time, which in some cases can reach about 2–3 h. LIB manufacturers recommend charging at 0.8 C or less to prolong battery life. In this case, the charging efficiency is about 99 percent, and the change in temperature conditions during charging is within the acceptable range. Some LIBs can withstand a temperature rise of 5 °C when fully charged. This could be due to protective circuitry and/or increased internal resistance. A full charge occurs when the battery reaches the threshold voltage and the current drops to three percent of its nominal value [58,59,60,61][10][11][12][13].
LIB do not need to be fully charged, as is the case with LAB. It is recommended not to allow the battery to be fully charged because the high voltage causes the battery to be unbalanced. Selecting a lower voltage threshold or eliminating the saturation charge completely extends battery life [62,63][14][15].
Another important distinguishing feature of LIB is its operation in a safe mode within a limited range of operating voltages. A long-term over-normalized charge forms a lithium metal coating on the anode, while the cathode material becomes an oxidizing element and becomes unstable, contributing to the formation of carbon dioxide (CO2). In this case, the pressure in the battery rises, and if the charge continues under current conditions, a protective device is triggered, which is responsible for the safe operation of the battery. If the pressure continues to build up, the diaphragm will rupture and eventually the battery could ignite. The critical temperatures of LIB for fully charged batteries are, depending on the technology used: for cobalt 130 °C–150 °C, nickel-manganese-cobalt 170 °C–180 °C, and manganese 250 °C. LIB is not the only battery that requires proper handling and organization of permissible operating conditions in order to increase explosion and fire safety. LAB, NMGB and NCB can also be hazardous if mishandled. Properly designed charging equipment is paramount in all battery systems [64,65][16][17].
The characteristics of accumulator battery depend on the chemical composition of the components, but, despite this, an equivalent selection of the main characteristics for the traction storage battery is required, since they affect the quality and service life of the traction power source as a whole. Table 1 shows the main characteristics that you need to be guided by when choosing the most preferred type of rechargeable batteries [66,67,68][18][19][20].
Table 1. Main comparative characteristics of different types of batteries [67].
Main comparative characteristics of different types of batteries [19].
Battery Parameters Lead Acid Nickel-Cadmium Nickel Metal Hydride Li-ion
Battery rated voltage, V 2 1.2 1.2 3.7
Specific energy consumption, Wh/kg30–40 40–60
Fast-charging process -30–80 +90–140
+ + Specific power, W/kg 180 150 250–1000 1800
Average charge time, hour more than 10
Ease of disposal - - + +
Shelf life is more than 3 years +8 6 2
+ - + Number of discharge/charge cycles (service life) 500–800 2000 800 2000
Memory effect - + + - Average self-discharge per month, % 4 twenty thirty 7
Average cost per kWh, $
Permissible recharge150 400–800 250 450
To determine the most preferred type of TCS, the following characteristics were selected [68,69][20][21]:
  • Compactness is a comparative characteristic that determines the weight and size properties to provide the specified parameters;
  • Fast-charging process—the ability of the battery to be charged with the maximum currents for it in less than 2.5 h;
  • Ease of disposal—the complexity of the technological process associated with the disposal or the impossibility of recovering useful chemical elements;
  • Memory effect—a reversible loss of capacity that occurs in some types of electric batteries when the recommended charging mode is violated, in particular, when recharging an incompletely discharged battery;
  • Permissible overcharge—a quantitative indication that determines the permissible value when the battery is charged over 100%;
  • Depth of discharge (DOD)—the real amount (of the declared) energy that the battery can give without increasing the temperature.
  • The distribution of quality indicators is shown in Table 2.
Table 2. Qualitative comparison of batteries [70].
Qualitative comparison of batteries [22].
Comparison Parameter Lead Acid Nickel-Cadmium Nickel Metal Hydride Li-ion
Compactness -
60–90 days
Not regulated
Based on the performed analysis of charging characteristics, quantitative and qualitative comparison of indicators of batteries of four different types, the choice of LIB as traction is due to the following properties and indicators:
  • high indicators of specific characteristics;
+
+
+
High
Average
Short Very low
Depth of discharge (DOD) 50%
  • high values of permissible charging and discharging currents;
50–80%
50–85%
80%
Service intervals
3–6 months
30–60 days
  • the ability to quickly charge;
  • no need for maintenance;
  • maximum service life;
  • low self-discharge readings;
  • lack of “memory effect”.
The only negative quality of LIB today is their high cost, although there have been certain successes in the direction of reducing the cost of LIB in recent years [71,72][23][24].

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